Device with small molecular thiophene compound

ABSTRACT

An electronic device comprising a semiconductor layer in contact with a number of electrodes, wherein the semiconductor layer includes a small molecular thiophene compound consisting of a plurality of thiophene units, each thiophene unit being represented by structure (A)  
                 
         wherein each thiophene unit is bonded at either or both of the second ring position and the fifth ring position, wherein there is at least one thiophene unit where R 1  is present at the third ring position or the fourth ring position, or at both the third ring position and the fourth ring position, wherein for any two adjacent thiophene units there is excluded the simultaneous presence of the same or different R 1  at the 3-position of one thiophene unit and at the 3′-position of the other thiophene unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

Beng S. Ong et al., U.S. application Ser. No. unavailable (Attorney FileNo. A3247-US-NP), filed on the same day as the present application,titled “DEVICE WITH SMALL MOLECULAR THIOPHENE COMPOUND HAVING DIVALENTLINKAGE.”

Ping Liu et al., U.S. application Ser. No. unavailable (Attorney FileNo. A3246-US-NP), filed on the same day as the present application,titled “PROCESSES TO PREPARE SMALL MOLECULAR THIOPHENE COMPOUNDS.”

BACKGROUND OF THE INVENTION

Organic semiconductor compounds are central to the low-costmanufacturing of organic thin film transistors (“TFTs”). However, manyorganic semiconductor compounds suffer from either difficulties inprocessing in solution and/or instability in ambient conditions. Inaddition, certain conventional synthetic processes for preparing theseorganic semiconductor compounds involve multi-step reaction routes ofrelatively low overall yields. Thus, there is a need addressed byembodiments of the present invention for organic TFTs which incorporateorganic semiconductor compounds that are solution processable and/orexhibit good stability in ambient environmental conditions. Furthermore,the present invention in embodiments provides a new process forpreparing certain organic semiconductor compounds that involves fewerreaction steps and provides a higher overall yield, as compared withconventional processes.

In the literature, the term “oligomer” may convey two differentdefinitions: one refers to a mixture of low-molecular weight compoundswhich consist of small numbers of repeating units of one or morechemical entities, and is therefore a subset of a polymer. The oligomerunder this definition is generally characterized by number-average andweight-average molecular weights. A polymer refers to a mixture of highmolecular-weight compounds consisting of large numbers of repeatingunits of one or more chemical entities. The distinction of low and highmolecular weights to distinguish oligomer and polymer has not beenclearly drawn. The other definition of “oligomer” refers to a lowmolecular-weight compound consisting of a specific number of repeatingunits of one or more chemical entities, and it is thereforecharacterized by a specific molecular weight. Every molecule of theoligomer under this definition is identical in all respects. We use theterm “small molecular compound” to describe this class of oligomers toavoid confusion.

The following documents provide background information:

Marks et al., WO 02/09201 A1.

Aratani et al., U.S. Pat. No. 5,705,826.

Tsuyoshi Izumi et al., “Synthesis and Spectroscopic Properties of aSeries of Beta-Blocked Long Oligothiophenes up to the 96 mer:Revaluation of Effective Conjugation Length,” J. Am. Chem. Soc., Vol.125, No. 18, pp. 5286-5287 and S1-S6 (Apr. 10, 2003).

Francis Garnier et al., “Molecular Engineering of OrganicSemiconductors: Design of Self-Assembly Properties in ConjugatedThiophene Oligomers,” J. Am. Chem. Soc., Vol. 115, No. 19, pp. 8716-8721(1993).

Howard Katz et al., “Synthesis, Solubility, and Field-Effect Mobility ofElongated and Oxa-Substituted alpha, omega-Dialkyl Thiophene Oligomers.Extension of ‘Polar Intermediate’ Synthetic Strategy and SolutionDeposition on Transistor Substrates,” Chem. Mater., Vol. 10, No. 2, pp.633-638 (1998).

V. M. Niemi et al., “Polymerization of 3-alkylthiophenes with FeCl₃ ,”Polymer, Vol. 33, No. 7, pp. 1559-1562 (1992).

A. Afzali et al., “An Efficient Synthesis of SymmetricalOligothiophenes: Synthesis and Transport Properties of a SolubleSexithiophene Derivative,” Chem. Mater., Vol. 14, No. 4, pp. 1742-1746(Mar. 6, 2002).

X. Michael Hong et al., “Thiophene-Phenylene and Thiophene-ThiazoleOligomeric Semiconductors with High Field-Effect Transistor On/OffRatios,” Chem. Mater., Vol. 13, No. 12, pp. 4686-4691 (2001).

Beng Ong et al., U.S. application Ser. No. 10/042,358 (Attorney DocketNo. D/A1332), filed Jan. 11, 2002, titled “POLYTHIOPHENES AND DEVICESTHEREOF,” which has been published as US Published Application2003/0160230.

Beng Ong et al., U.S. application Ser. No. 10/042,342 (Attorney DocketNo. D/A1333), filed Jan. 11, 2002, titled “POLYTHIOPHENES AND DEVICESTHEREOF,” which has been published as US Published Application2003/0160234.

Beng Ong et al., U.S. application Ser. No. 10/042,356 (Attorney DocketNo. D/A1334), filed Jan. 11, 2002, titled “POLYTHIOPHENES AND DEVICESTHEREOF,” which issued as U.S. Pat. No. 6,621,099.

SUMMARY OF THE DISCLOSURE

In embodiments, there is provided an electronic device comprising asemiconductor layer in contact with a number of electrodes, wherein thesemiconductor layer includes a small molecular thiophene compoundconsisting of a plurality of thiophene units, each thiophene unit beingrepresented by structure (A)

-   -   wherein each thiophene unit is bonded at either or both of the        second ring position and the fifth ring position,    -   wherein m is 0, 1, or 2,    -   wherein each thiophene unit is the same or different from each        other in terms of substituent number, substituent identity, and        substituent position,    -   wherein each R₁ is independently selected from the group        consisting of:        (a) a hydrocarbon group,        (b) a heteroatom containing group, and        (c) a halogen,    -   wherein there is at least one thiophene unit where R₁ is present        at the third ring position or the fourth ring position, or at        both the third ring position and the fourth ring position,    -   wherein for any two adjacent thiophene units as represented by        structure (A1):    -   there is excluded the simultaneous presence of the same or        different R₁ at the 3-position of one thiophene unit and at the        3′-position of the other thiophene unit.

In further embodiments, there is provided a composition comprising: asmall molecular thiophene compound consisting of a plurality ofthiophene units, each thiophene unit being represented by structure (A)

-   -   wherein each thiophene unit is bonded at either or both of the        second ring position and the fifth ring position,    -   wherein m is 0, 1, or 2,    -   wherein each thiophene unit is the same or different from each        other in terms of substituent number, substituent identity, and        substituent position,    -   wherein each R₁ is independently selected from the group        consisting of:        (a) a hydrocarbon group,        (b) a heteroatom containing group, and        (c) a halogen,    -   wherein there is at least one thiophene unit where R₁ is present        at the third ring position or the fourth ring position, or at        both the third ring position and the fourth ring position,    -   wherein for any two adjacent thiophene units as represented by        structure (A1):    -   there is excluded the simultaneous presence of the same or        different R₁ at the 3-position of one thiophene unit and at the        3′-position of the other thiophene unit.

In other embodiments, there is provided an electronic device comprisinga semiconductor layer in contact with a number of electrodes, whereinthe semiconductor layer includes a small molecular thiophene compoundconsisting of:

-   -   at least one divalent linkage; and    -   a plurality of thiophene units, each thiophene unit being        represented by structure (A)    -   wherein each thiophene unit is bonded at either or both of the        second ring position and the fifth ring position,    -   wherein m is 0, 1, or 2,    -   wherein each thiophene unit is the same or different from each        other in terms of substituent number, substituent identity, and        substituent position,    -   wherein each R₁ is independently selected from the group        consisting of:        (a) a hydrocarbon group,        (b) a heteroatom containing group, and        (c) a halogen,    -   wherein there is at least one thiophene unit where R₁ is present        at the third ring position or the fourth ring position, or at        both the third ring position and the fourth ring position,    -   wherein for any two adjacent thiophene units as represented by        structure (A1):        there is excluded the simultaneous presence of the same or        different R₁ at the 3-position of one thiophene unit and at the        3′-position of the other thiophene unit, and    -   wherein the number of the thiophene units is at least 6.

In additional embodiments, there is provided a composition comprising asmall molecular thiophene compound consisting of:

-   -   at least one divalent linkage; and    -   a plurality of thiophene units, each thiophene unit being        represented by structure (A)    -   wherein each thiophene unit is bonded at either or both of the        second ring position and the fifth ring position,    -   wherein m is 0, 1, or 2,    -   wherein each thiophene unit is the same or different from each        other in terms of substituent number, substituent identity, and        substituent position,    -   wherein each R₁ is independently selected from the group        consisting of:        (a) a hydrocarbon group,        (b) a heteroatom containing group, and        (c) a halogen,    -   wherein there is at least one thiophene unit where R₁ is present        at the third ring position or the fourth ring position, or at        both the third ring position and the fourth ring position,    -   wherein for any two adjacent thiophene units as represented by        structure (A1):        there is excluded the simultaneous presence of the same or        different R₁ at the 3-position of one thiophene unit and at the        3′-position of the other thiophene unit, and    -   wherein the number of the thiophene units is at least 6.

More embodiments include a process comprising:

-   -   subjecting a reaction mixture comprising a reaction medium, a        coupling agent, and a precursor to a coupling temperature to        preferentially form a desired small molecular thiophene compound        in a single-step synthesis,    -   wherein the precursor consists of:        (i) an optional divalent linkage, and        (ii) a plurality of thiophene units, each thiophene unit being        represented by structure (A)    -   wherein each thiophene unit is bonded at either or both of the        second ring position and the fifth ring position,    -   wherein m is 0, 1, or 2,    -   wherein each thiophene unit is the same or different from each        other in terms of substituent number, substituent identity, and        substituent position,    -   wherein each R₁ is independently selected from the group        consisting of:        (a) a hydrocarbon group,        (b) a heteroatom containing group, and        (c) a halogen.

Additional embodiments include a process comprising:

-   -   subjecting a reaction mixture comprising a reaction medium, a        coupling agent, and a precursor to a coupling temperature to        preferentially form a desired small molecular thiophene compound        in a single-step synthesis, wherein precipitation in the        reaction mixture spontaneously occurs and the precipitate        includes the desired small molecular thiophene compound,    -   wherein the precursor consists of:        (i) an optional divalent linkage, and        (ii) a plurality of thiophene units, each thiophene unit being        represented by structure (A)    -   wherein each thiophene unit is bonded at either or both of the        second ring position and the fifth ring position,    -   wherein m is 0, 1, or 2,    -   wherein each thiophene unit is the same or different from each        other in terms of substituent number, substituent identity, and        substituent position,    -   wherein each R₁ is independently selected from the group        consisting of:        (a) a hydrocarbon group,        (b) a heteroatom containing group, and        (c) a halogen.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects of the present invention will become apparent as thefollowing description proceeds and upon reference to the followingfigures which represent exemplary embodiments:

FIG. 1 represents a first embodiment of the present invention in theform of a thin film transistor;

FIG. 2 represents a second embodiment of the present invention in theform of a thin film transistor;

FIG. 3 represents a third embodiment of the present invention in theform of a thin film transistor; and

FIG. 4 represents a fourth embodiment of the present invention in theform of a thin film transistor.

Unless otherwise noted, the same reference numeral in different Figuresrefers to the same or similar feature.

DETAILED DESCRIPTION

The term “molecular” in “small molecular thiophene compound” indicatesthat the compound has a specific number (not an average number) ofthiophene units. In embodiments, the small molecular thiophene compoundhas a purity of at least about 90% by weight, or at least about 98% byweight. While impurities (e.g., reaction by-product thiophenecompound(s) with a different number of thiophene units) may be present,the small molecular thiophene compound remains that of a compound havinga specific number, not an average number, of thiophene units. Inembodiments, the purity of the small molecular thiophene compound may bedescribed by terms denoting a purity level where exemplary terms are forexample “ACS Reagent Grade” (for example ≧95% by weight), “HPLC Grade”(for example ≧99.9% by weight) and “Semiconductor Grade” (for example≧99.99% by weight). In embodiments, the small molecular thiophenecompound has the best obtainable purity. Before isolated from thereaction mixture, the small molecular thiophene compound is referred toas the “desired small molecular thiophene compound.”

Unless otherwise noted, both the “small molecular thiophene compound”and the “desired small molecular thiophene compound” are referred hereinas the “compound.”

The term “small” in “small molecular thiophene compound” and in “desiredsmall molecular thiophene compound” indicates a small number ofthiophene units and is intended to distinguish from a structure having alarge number of units such as a polymer with many repeating units. Inembodiments, the compound has a specific number of thiophene units ofstructure (A) ranging from about 4 to about 25, or from about 5 to about20. Values outside these ranges are encompassed by embodiments of thepresent compound as long as the number of thiophene units in thecompound is consistent with the meaning of “small.”

The compound described herein is a thiophene derivative and is composedof an optional divalent linkage or linkages and a specific number ofthiophene units of structure (A) which are either mono- or divalentlybonded to each other, or to the optional divalent linkage, in thecompound. Each thiophene unit is of structure (A)

where R₁ is independently selected from a hydrocarbon group, aheteroatom containing group, and a halogen, and where m is 0, 1, or 2.

The small molecular thiophene compound may be prepared by a controlledcoupling reaction of a thiophene precursor, which is composed of anumber of covalently linked thiophene units of structure (A) and anoptional divalent linkage or linkages, to provide a small molecularcompound composed of a specific number of thiophene units of structure(A) and the optional divalent linkage or linkages.

In certain embodiments of the compound, for any two adjacent thiopheneunits within the structure of the compound, there is excluded thesimultaneous presence of substituents at the 3, 3′ positions, that is tosay, no simultaneous substitutions at the third ring positions (3 and3′) of two adjacent thiophene units. In embodiments, molecules of thecompound may exhibit extensive pi-conjugation, but the simultaneouspresence of substituents at the third ring positions may cause torsionaldeviation of the two adjacent thiophene units from coplanarity, thussignificantly breaking down the pi-conjugation of the molecules. Shortpi-conjugation length may lead to short pi-delocalization and thus poorchange carrier transport capability and low mobility. It is understoodthat two thiophene units with an intervening divalent linkage are notconsidered adjacent. To illustrate the meaning of no simultaneoussubstitutions at the third ring positions of two adjacent thiopheneunits, the relevant ring positions of each thiophene unit is identifiedin a two thiophene segment represented by structure (A1):

-   -   where the third ring positions of the thiophene units are        identified as 3 and 3′. It is noted that the discussion        regarding no simultaneous substitutions at the third ring        positions of two adjacent thiophene units does not exclude        embodiments where for each thiophene unit in the two thiophene        unit segment, R₁ forms part of a ring structure attached to the        carbon atoms at the third ring position and the fourth ring        position of the thiophene unit. The fused cyclic R₁ substituent        on each of the two adjacent thiophenes may be the same or        different from one another. In embodiments, the simultaneous        presence of fused cyclic R₁ substituents at the third ring        positions does not appear to cause significant torsional        deviation of the two adjacent thiophene units from coplanarity,        thus still capable of maintaining substantial pi-conjugation of        the molecules. In structure A1, it is understood that the phrase        two adjacent thiophene units also encompasses embodiments where        one of the thiophene units is a “terminal” thiophene unit.

Optionally, the compound may contain in its structure one or moredivalent linkages such as for example those represented by the followingstructures:

-   -   wherein n is 0, 1, 2, 3, or 4, and the substituents of R₄ are        the same or different from each other within each divalent        linkage and among different divalent linkages. R₄ may be a        hydrocarbon group, a heteroatom containing group, and a halogen.

If present, any suitable number of the same or different divalentlinkage may be present in the compound such as from 1 to about 5 for thecompound.

In embodiments of the compound, regardless whether the two “terminal”thiophene units (i.e., the thiophene unit located at each end of thecompound) have any R₁ substituent, there is at least one “internal”thiophene unit (i.e., those thiophene units other than the “terminal”thiophene units) where R₁ is present at the third ring position or thefourth ring position, or at both the third ring position and the fourthring position. The presence of the R₁ substituent at the “internal”thiophene unit(s) may help induce and facilitate intermolecularinteractions through interaction of R₁ substituents, and this will allowmolecules to self-organize into proper molecular ordering. Propermolecular ordering is conducive to charge carrier transport.

In embodiments of the present invention, one or both of the followingaspects are optional:

-   -   (a) for the small molecular thiophene compound there is at least        one thiophene unit where R₁ is present at the third ring        position or the fourth ring position, or at both the third ring        position and the fourth ring position; and    -   (b) wherein for any two adjacent thiophene units in the small        molecular thiophene compound, there is excluded the simultaneous        presence of the same or different R₁ at the third ring position        of one thiophene unit and at the third ring position of the        other thiophene unit.

Exemplary compounds without the divalent linkage are for example thefollowing:

-   -   where y, R, and R′ are described herein.

In specific embodiments, the compounds can be:

Exemplary compounds with the divalent linkage are for example thefollowing:

-   -   where y, R, and R′ are described herein.

In specific embodiments, the compounds with the divalent linkage can be:

The total number of thiophene units of structure (A) in the compound(the optional divalent linkage may be present) may be an even or oddnumber and may be for example: at least 4, at least 6, at least 8, atleast 10, from 6 to 32, from 8 to 32, from 6 to 20, or from 8 to 20. Inembodiments, the number of the thiophene units in the small molecularthiophene compound is selected from the group consisting of 6, 8, 12,and 16. In embodiments, a larger number of thiophene units is preferredbecause longer pi-conjugation lengths lead to extendedpi-delocalization, which is more favorable for charge carrier transport.

The small molecular thiophene compound can be synthesized by anysuitable reactions. In embodiments, the small molecular thiophenecompound can be synthesized by a coupling reaction of a precursorcompound composed of a number of thiophene units of structure (A) and anoptional divalent linkage or linkages. Exemplary precursors without thedivalent linkage are for example the following:

-   -   where R is described herein.

Exemplary precursors with the divalent linkage are for example thefollowing:

-   -   where R is described herein.

The number of thiophene units of structure (A) in the precursor (theoptional divalent linkage may be present) may range for example from 2to 8, 3 to 8, or from 3 to 6.

Possible substituents for all the recited herein, structures (A), (A2)through (A10), and (B1) through (B7) are now discussed in more detail.Unless otherwise noted, the possible substituents apply to all thiopheneunits in each structure.

Number of Units of y

In embodiments, y is the number of units ranging for example from about2 to about 6, or from about 2 to about 4.

Substituent R

R and R′ are the same or different from each other where R is selectedfrom the group consisting of:

-   -   (a) a hydrocarbon group,    -   (b) a heteroatom containing group, and    -   (c) a halogen.

Substituent R′

R′ is selected from the group consisting of:

-   -   (a) a hydrocarbon group,    -   (b) a heteroatom containing group,    -   (c) a halogen, and    -   (d) a hydrogen.

Hydrocarbon Group for R, R′, R₁, R₂, R₃, R₄

The hydrocarbon group contains for example from 1 to about 25 carbonatoms, or from 1 to about 10 carbon atoms, and may be for example astraight chain alkyl group, a branched alkyl group, a cycloalkyl group,an aryl group, an alkylaryl group, and an arylalkyl group. Exemplaryhydrocarbon groups include for example methyl, ethyl, propyl, butyl,pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl,tetradecyl, pentadecyl, cyclopentyl, cyclohexyl, cycloheptyl, andisomers thereof.

The hydrocarbon group is optionally substituted one or more times withfor example a halogen (chlorine, bromine, fluorine, and iodine).

Heteroatom Containing Group for R, R′, R₁, R₂, R₃, R₄

The heteroatom containing group has for example 2 to about 50 atoms, orfrom 2 to about 30 atoms) and may be for example a nitrogen containingmoiety, an alkoxy group, a heterocyclic system, an alkoxyaryl, and anarylalkoxy. Exemplary heteroatom containing groups include for examplecyano, nitro, methoxyl, ethoxyl, and propoxy.

R₁ is Cyclic Substituent Fused to Thiophene Unit

In embodiments, R₁ is part of a cyclic ring structure fused to thethiophene unit, where the fused cyclic ring structure is of any sizesuch as for example a 4 to 8 membered ring, particularly, a 5 or 6membered ring, wherein R₁ is attached at the carbon atoms at the thirdring position and the fourth ring position of the thiophene unit. Thefused cyclic ring structure (containing R₁) may be either a hydrocarbongroup described herein or a heteroatom containing group describedherein. Where R₁ is part of a ring structure fused to a thiophene unit,m is 1 even though R₁ in this situation is bonded to two positions ofthe thiophene unit. Examples of the thiophene unit with R₁ being part ofa ring substituent structure are the following:

-   -   wherein R₂ and R₃ are the same or different from each other, and        are selected from the group consisting of:    -   (a) a hydrocarbon group,    -   (b) a heteroatom containing group,    -   (c) a halogen, and    -   (d) a hydrogen.

Halogen for R, R′, R₁, R₂, R₃, R₄

The halogen may be chlorine, bromine, fluorine, and iodine.

The compound is symmetrical or unsymmetrical. The term symmetricalrefers to a structure that exhibits a regular repeated pattern of thecomponent parts. The term symmetrical also refers to two component partsof the compound in a plane such that the line segment joining the twocomponent parts is bisected by an axis, a point or a center. On theother hand, unsymmetrical compounds refer to the compounds lacking ofthe above-mentioned symmetry. An example of a symmetrical compound is anembodiment of compound (A2) where the two R moieties are the same andthe two R′ moieties are the same. An example of an unsymmetricalcompound is an embodiment of compound (A2) where the two R moieties aredifferent or where the two R′ moieties are different.

The compound may be a p-type semiconductor compound or a n-typesemiconductor compound, depending on the substituents. In general,substituents with electron-donating property such as a hydrocarbon likealkyl, alkyloxy and phenylene groups will make the moleculeelectron-rich, thus turning the molecule into p-type; while substituentswith electron-withdrawing ability such as cyano, nitro, fluoro, andfluorinated alkyl groups will make the thiophene moleculeelectron-deficient, thus turning the compound into a n-typesemiconductor.

In embodiments, a composition composed of two or more of different smallmolecular thiophene compounds may be used for the fabrication of thesemiconductor layers for thin film transistors. In the composition, thesmall molecular thiophene compounds are physically added together in apredetermined ratio, thus controlling both the number of compounds andtheir ratio in the composition. In the composition, the different smallmolecular thiophene compounds may be present in any suitable amountssuch as for example ranging from about 5% (first molecular compound):95%(second molecular compound) by weight to 95% (first molecularcompound):5% (second molecular compound) by weight. Each small molecularthiophene compound in the composition may be synthesized as describedherein.

Preparation of the Compound Via Oxidative Coupling Reaction of Precursor

One exemplary technique to synthesize the compound involves subjectingto a suitable coupling temperature (e.g., an elevated temperature fromheating or room temperature) a reaction mixture comprising a reactionmedium (a single reaction medium or a mixture of two or more differentreaction media in any suitable ratio), a coupling agent (a singlecoupling agent or a mixture of two more different coupling agents in anysuitable ratio), and a precursor (a single precursor or a mixture of twoor more different precursors in any suitable ratio). Subjecting thereaction mixture to the coupling temperature causes the precursor toself-couple under the influence of coupling agent, resulting in thepreferential formation of the desired small molecular thiophene compoundwith the desired number of thiophene units in the compound. The phrases“preferential formation” and “preferentially form” indicate that in thereaction mixture after a certain predetermined reaction time, thedesired small molecular thiophene compound has the highest concentrationby weight compared to any single one of the reaction byproduct thiophenecompound(s), where the desired small molecular thiophene compound ispresent in an amount ranging from about 30% to about 90%, or from about40% to about 80%, by weight based on the weight of the reaction mixture(in embodiments, these percentages are based on the weight of theprecipitate if precipitation occurs). The phrase “reaction byproductthiophene compound(s)” encompasses any molecules of unreacted precursorand any thiophene compound(s) different from the desired small molecularthiophene compound.

In embodiments, subjecting the reaction mixture to the couplingtemperature will within a period of time “spontaneously” cause aprecipitate (as observed by the naked eye) to form which includes thedesired small molecular thiophene compound and optionally any reactionbyproduct thiophene compound(s). The term “spontaneously” means that theprecipitate will come out of the reaction medium in due course withoutthe need for any additional procedure(s) to facilitate theprecipitation. In other embodiments of the present invention, one ormore additional procedure(s) may be optionally undertaken to hasten theprecipitation, to increase the amount of precipitate, or to increase theproportion of the desired small molecular thiophene compound in thereaction mixture and/or precipitate, such as by decreasing thesolubility of the desired small molecular thiophene compound in thereaction medium by adding a different reaction medium selected for thatpurpose. Even with the use of these optional additional procedures, thepresent process is still considered to “spontaneously” cause theprecipitate to form since precipitation would occur in the absence ofthese additional procedures. In other embodiments, no precipitate isformed and the desired small molecular thiophene compound which isformed is soluble in the reaction medium.

In embodiments, the reaction medium and the coupling temperature areselected to cause precipitation “spontaneously” of the desired smallmolecular thiophene compound when the concentration of the desired smallmolecular thiophene compound reaches a saturation point in the reactionmedium. The saturation point of the desired small molecular thiophenecompound is dictated by its solubility in the reaction medium, and thusvaries depending on the nature of the desired small molecular thiophenecompound, the reaction medium, and the temperature of the reactionmedium. Of all the molecules of the desired small molecular thiophenecompound in the reaction mixture, all or a portion thereof mayprecipitate, such as from about 30% to 100% by weight.

The formation of the desired small molecular thiophene compound occursin a single-step synthesis. The phrase “single-step synthesis” refers toa synthesis route of one reaction in contrast to a “multi-stepsynthesis” involving two or more reactions to synthesize the desiredsmall molecular thiophene compound. The distinction between “single-stepsynthesis” and multi-step synthesis” is understood by those of ordinaryskill in the art. In embodiments of the “single-step synthesis,” anintermediate product may be formed in the reaction mixture where theintermediate product undergoes further reaction in the reaction mixtureto form the desired small molecular thiophene compound. In embodiments,one or more additional optional procedures may be undertaken to hastenthe precipitation, to increase the amount of precipitate, or to increasethe proportion of the desired small molecular thiophene compound in thereaction mixture and/or precipitate, but these additional procedures arenot considered a synthesis reaction and thus the present process remainsa “single-step synthesis” even with the optional use of these additionalprocedures. Similarly, any procedures used to isolate the desired smallmolecular thiophene compound from the reaction mixture or theprecipitate are not a synthesis reaction and thus the present processremains a “single-step synthesis” even with the use of isolationprocedures.

The coupling temperature can be maintained at the same temperature orvaried over time to for example control the proportion of the desiredsmall molecular thiophene compound in the reaction mixture and/orprecipitate. The reaction mixture is subjected to the couplingtemperature for a time ranging for example from about 10 minutes toabout 24 hours, or from about 1 hour to about 5 hours. The couplingtemperature ranges for example from about room temperature to about 150degrees C., or from about 23 to about 150 degrees C., or from about 30to about 140 degrees C., or from about 50 to about 80 degrees C. As usedherein, room temperature refers to a temperature ranging for examplefrom about 23 to about 25 degrees C.

In embodiments, prior to any heating of the reaction mixture, the amountof precursor dissolved in the reaction medium at room temperature mayrange for example from about 0.001% to about 75% by weight, or fromabout 0.025% to about 50% by weight, based on the weight of theprecursor. The precursor(s) employed may be any of those describedherein composed of the thiophene units of structure (A) and the optionaldivalent linkage. Exemplary concentrations of the precursor are from0.001% to about 50% by weight, more specifically, 0.025% to about 30% byweight based on the total reaction medium and precursor.

The reaction medium may be for example tetrahydrofuran, toluene,chloroform dichloromethane, chlorobenzene, dichlorobenzene,dichloroethane, 1,2-dichloroethane, xylene, heptane, mesitylene,nitrobenzene, acetonitrile, cyanobenzene, or a mixture thereof. Inembodiments, the reaction medium is considered a solvent.

The coupling agent may be for example FeCl₃, RuCl₃, MoCl₅, or a mixturethereof. The molar ratio of coupling agent to precursor is for examplefrom 1:1 to 10:1, particularly 2:1 to 6:1. In embodiments, the couplingagent is an oxidative agent.

The desired small molecular thiophene compound can be isolated from theprecipitate using any suitable technique such as by columnchromatography.

In embodiments, the desired small molecular thiophene compound stays insolution after its preferential formation in the reaction mixture anddoes not spontaneously precipitate at the coupling temperature. In suchembodiments, the desired small molecular thiophene compound can beisolated from the reaction mixture by any suitable procedures such asfor example evaporation of all solvents, addition of poor solvent intothe reaction mixture resulting in precipitation of the desired smallmolecular thiophene compound, extraction with different solvent, anddirectly running a column chromotrography.

In embodiments, the present invention may be used whenever there is aneed for a semiconductor layer in an electronic device. The phrase“electronic device” refers to macro-electronic devices such as solarcell devices and to micro- and nano-electronic devices such as, forexample, micro- and nano-sized transistors and diodes. Illustrativetransistors include for instance thin film transistors, particularlyfield effect transistors.

Any suitable techniques may be used to form the semiconductor layercontaining the compound. One such method is by vacuum evaporation at avacuum pressure of about 10⁻⁵ to 10⁻⁷ torr in a chamber containing asubstrate and a source vessel that holds the compound in powdered form.Heat the vessel until the compound sublimes onto the substrate. Theperformance of the films containing the compound depends on the rate ofheating, the maximum source temperature and/or substrate temperatureduring process. In embodiments, solution deposition techniques may alsobe used to fabricate a thin film containing the compound. The phrase“solution deposition techniques” refers to any liquid depositiontechnique such as spin coating, blade coating, rod coating, screenprinting, ink jet printing, stamping and the like. Specifically, thecompound is dissolved in a suitable liquid of for exampletetrehydrofuran, dichlorormethane, chlororbenzene, toluene, and xyleneat a concentration of about 0.1% to 10%, particularly 0.5% to 5% byweight, followed by spin coating at a speed of about 500 to 3000 rpm,particularly 1000-2000 rpm for a period of time of about 5 to 100seconds, particularly about 30 to 60 seconds at room temperature or anelevated temperature.

In FIG. 1, there is schematically illustrated a thin film transistor(“TFT”) configuration 10 comprised of a substrate 16, in contacttherewith a metal contact 18 (gate electrode) and a layer of aninsulating layer 14 on top of which two metal contacts, source electrode20 and drain electrode 22, are deposited. Over and between the metalcontacts 20 and 22 is an organic semiconductor layer 12 as illustratedherein.

FIG. 2 schematically illustrates another TFT configuration 30 comprisedof a substrate 36, a gate electrode 38, a source electrode 40 and adrain electrode 42, an insulating layer 34, and an organic semiconductorlayer 32.

FIG. 3 schematically illustrates a further TFT configuration 50comprised of a heavily n-doped silicon wafer 56 which acts as both asubstrate and a gate electrode, a thermally grown silicon oxideinsulating layer 54, and an organic semiconductor layer 52, on top ofwhich are deposited a source electrode 60 and a drain electrode 62.

FIG. 4 schematically illustrates an additional TFT configuration 70comprised of substrate 76, a gate electrode 78, a source electrode 80, adrain electrode 82, an organic semiconductor layer 72, and an insulatinglayer 74.

The composition and formation of the semiconductor layer are describedherein.

The substrate may be composed of for instance silicon, glass plate,plastic film or sheet. For structurally flexible devices, plasticsubstrate, such as for example polyester, polycarbonate, polyimidesheets and the like may be preferred. The thickness of the substrate maybe from amount 10 micrometers to over 10 millimeters with an exemplarythickness being from about 50 to about 100 micrometers, especially for aflexible plastic substrate and from about 1 to about 10 millimeters fora rigid substrate such as glass or silicon.

The compositions of the gate electrode, the source electrode, and thedrain electrode are now discussed. The gate electrode can be a thinmetal film, a conducting polymer film, a conducting film made fromconducting ink or paste or the substrate itself, for example heavilydoped silicon. Examples of gate electrode materials include but are notrestricted to aluminum, gold, chromium, indium tin oxide, conductingpolymers such as polystyrene sulfonate-dopedpoly(3,4-ethylenedioxythiophene) (PSS-PEDOT), conducting ink/pastecomprised of carbon black/graphite or colloidal silver dispersion inpolymer binders, such as ELECTRODAG™ available from Acheson ColloidsCompany. The gate electrode layer can be prepared by vacuum evaporation,sputtering of metals or conductive metal oxides, coating from conductingpolymer solutions or conducting inks by spin coating, casting orprinting. The thickness of the gate electrode layer ranges for examplefrom about 10 to about 200 nanometers for metal films and in the rangeof about 1 to about 10 micrometers for polymer conductors. The sourceand drain electrode layers can be fabricated from materials whichprovide a low resistance ohmic contact to the semiconductor layer.Typical materials suitable for use as source and drain electrodesinclude those of the gate electrode materials such as gold, nickel,aluminum, platinum, conducting polymers and conducting inks. Typicalthicknesses of source and drain electrodes are about, for example, fromabout 40 nanometers to about 1 micrometer with the more specificthickness being about 100 to about 400 nanometers.

The insulating layer generally can be an inorganic material film or anorganic polymer film. Illustrative examples of inorganic materialssuitable as the insulating layer include silicon oxide, silicon nitride,aluminum oxide, barium titanate, barium zirconium titanate and the like;illustrative examples of organic polymers for the insulating layerinclude polyesters, polycarbonates, poly(vinyl phenol), polyimides,polystyrene, poly(methacrylate)s, poly(acrylate)s, epoxy resin and thelike. The thickness of the insulating layer is, for example from about10 nanometers to about 500 nanometers depending on the dielectricconstant of the dielectric material used. An exemplary thickness of theinsulating layer is from about 100 nanometers to about 500 nanometers.The insulating layer may have a conductivity that is for example lessthan about 10⁻¹² S/cm.

In embodiments, the insulating layer, the gate electrode, thesemiconductor layer, the source electrode, and the drain electrode areformed in any sequence with the gate electrode and the semiconductorlayer both contact the insulating layer, and the source electrode andthe drain electrode both contact the semiconductor layer. The phrase “inany sequence” includes sequential and simultaneous formation. Forexample, the source electrode and the drain electrode can be formedsimultaneously or sequentially. The composition, fabrication, andoperation of field effect transistors are described in Bao et al., U.S.Pat. No. 6,107,117, the disclosure of which is totally incorporatedherein by reference.

The semiconductor layer has a thickness ranging for example from about10 nanometers to about 1 micrometer with a preferred thickness of fromabout 20 to about 200 nanometers. The TFT devices contain asemiconductor channel with a width W and length L. The semiconductorchannel width may be, for example, from about 1 micrometers to about 5millimeters, with a specific channel width being about 5 micrometers toabout 1 millimeter. The semiconductor channel length may be, forexample, from about 1 micrometer to about 1 millimeter with a morespecific channel length being from about 5 micrometers to about 100micrometers.

The source electrode is grounded and a bias voltage of generally, forexample, about 0 volt to about −80 volts is applied to the drainelectrode to collect the charge carriers transported across thesemiconductor channel when a voltage of generally about +20 volts toabout −80 volts is applied to the gate electrode.

Regarding electrical performance characteristics, a semiconductor layerof the present electronic device has a carrier mobility greater than forexample about 10⁻³ cm²/Vs (centimeters²/Volt-second) and a conductivityless than for example about 10⁻⁴ S/cm (Siemens/centimeter). The thinfilm transistors produced by the present process have an on/off ratiogreater than for example about 10³ at 20 degrees C. The phrase on/offratio refers to the ratio of the source-drain current when thetransistor is on to the source-drain current when the transistor is off.

The invention will now be described in detail with respect to specificexemplary embodiments thereof, it being understood that these examplesare intended to be illustrative only and the invention is not intendedto be limited to the materials, conditions, or process parametersrecited herein. All percentages and parts are by weight unless otherwiseindicated. Where provided in the Examples, the NMR spectra were recordedat room temperature using a Bruker DPX 300 NMR spectrometer

EXAMPLE 1

(i) Synthesis of Small Molecular Thiophene Compound (1)

The preparation of precursor,5,5′-bis(3-dodecyl-2-thienyl)-2,2′-dithiophene, (10), is illustrated inScheme 1.

A solution of 2-bromo-3-dodecylthiophene (15.36 grams, 46.36 mmol) in 40milliliters of anhydrous tetrahydrofuran (THF) was added slowly over aperiod of 20 minutes to a mechanically stirred suspension of magnesiumturnings (1.69 grams, 69.50 mmol) in 5 milliliters of anhydrous THF in a250 milliliter round-bottomed flask under an inert argon atmosphere.When reaction was initiated, the reaction mixture was stirred at 60° C.for 3 hours before cooling down to room temperature. The resultantmixture was then added via a cannula to a mixture of5,5′-dibromo-2,2′-dithiophene (6.01 grams, 18.54 mmol) and[1,2-bis(diphenylphosphino)ethane]dichloronickel (II) (0.37 gram of(dppe)NiCl₂, 0.70 mmol) in 80 milliliters of anhydrous THF in a 250milliliter round-bottomed flask under an argon atmosphere, and thenrefluxed for 48 hours. Subsequently, the reaction mixture was cooleddown to room temperature and washed with water. The crude product wasextracted with ethyl acetate and dried with anhydrous sodium sulfate. Adark brown syrup, obtained after evaporation of the solvent, waspurified by column chromotography on silica gel to yield crude5,5′-bis(3-dodecyl-2-thienyl)-2,2′-dithiophene (10), which wasrecrystallized from a mixture of dichloromethane(10 ml), isopropanol(250ml) and methanol (100 ml), yielding a yellow crystalline product in 66percent yield, m.p. 58.9° C.

The NMR spectrum of the above obtained compound was recorded at roomtemperature using a Bruker DPX 300 NMR spectrometer:

¹H NMR (CDCl₃): δ 7.18 (d, J=5.4 Hz, 2H), 7.13 (d, J=3.6 Hz, 2H), 7.02(d, J=3.6 Hz, 2H), 6.94 (d, J=5.4 Hz, 2H), 2.78 (t, 4H), 1.65 (q, 1.65,4H), 1.28 (bs, 36H), 0.88 (m, 6H). ¹³C NMR (CDCl₃, ppm): δ 139.78,136.73, 135.26, 130.26, 129.99, 126.43, 123.75, 123.71, 31.86, 30.59,29.62, 29.61, 29.54, 29.46, 29.40, 29.30, 29.20, 22.63, 14.05.

The oxidative coupling reaction of precursor (10) to give Compound (I)was conducted as follows: A solution of precursor (10) in 5 mlchlorobenzene was added slowly to a mixture of 0.4 g of FeCl₃ and 2milliliters of chloroform in a 50-milliliter reaction flask. Theresulting mixture was stirred at 40° C. for 1 hour, then at roomtemperature for 16 hours, and finally at 50° C. for 7 hours. After thereaction mixture was cooled down to the room temperature, it was pouredinto 50 milliliter of dichloromethane, and washed with water. Theorganic phase was separated and stirred with 200 milliliters of aqueous7.5% ammonia solution for 30 min. The organic phase was washed againwith water and then poured into stirring methanol. The precipitatedproduct was collected by filtration and dried under vacuo at roomtemperature overnight. Compound (I) was isolated from the crude productby column chromatography on silica gel using an eluent solvent systemconsisting of 95:5 of hexane:methylene chloride by volume, andrecrystallized from isopropanol as a red crystal in 52% yield, m.p.,78.70° C.

¹H-NMR(CDCl₃, ppm) δ 7.21-6.95 (m, 14H), δ 2.83-2.76 (m, 8H), δ1.77-1.60 (m, 8H), 1.50-1.14 (bs, 72H), 0.97-0.85 (m, 12H).

¹³C NMR (CDCl₃, ppm): δ 140.98, 140.32, 137.23, 137.09, 135.84, 135.36,135.27, 130.53, 129.89, 127.02, 126.94, 126.74, 124.28, 32.33, 31.07,30.86, 30.10, 30.08, 30.01, 29.93, 29.87, 29.77, 29.68, 23.10, 14.53.

(ii) Device Fabrication and Evaluation

A bottom-contact thin-film transistor structure as schematicallyillustrated by FIG. 1, was used as the test device configuration. Thetest device was comprised of a series of photolithographicallypre-patterned transistor dielectric layers and electrodes with definedchannel widths and lengths on a glass substrate. The gate electrode wascomprised of chromium of about 80 nanometers in thickness. Theinsulating layer was a 300 nanometers thick silicon nitride having acapacitance of about 22 nF/cm² (nanofarads/square centimeter). On top ofsaid insulating layer were coated by vacuum deposition the source anddrain contacts comprised of gold of about 100 nanometers in thickness.The semiconductor layer of about 50 nanometers to 100 nanometers inthickness was then deposited by spin coating a 1 wt % chloroformsolution of compound (I). The spin coating was accomplished at aspinning speed of 1,000 rpm for about 35 seconds. The resulting coateddevice was dried in vacuo at 60° C. for 20 hours, and was then ready forevaluation.

The evaluation of transistor performance was accomplished in a black boxat ambient conditions using a Keithley 4200 SCS semiconductorcharacterization system. The carrier mobility, μ, was calculated fromthe data in the saturated regime (gate voltage, V_(G)<source-drainvoltage, V_(SD)) accordingly to equation (1)I _(SD) =C _(i)μ(W/2L)(V _(G) −V _(T))²  (1)

-   -   where I_(SD) is the drain current at the saturated regime, W and        L are, respectively the semiconductor channel width and length,        C_(i) is the capacitance per unit area of the insulating layer,        and V_(G) and V_(T) are respectively the gate voltage and        threshold voltage. V_(T) of the device was determined from the        relationship between the square root of I_(SD) at the saturated        regime and V_(G) of the device by extrapolating the measured        data to I_(SD)=0.

An important property for the thin film transistor is its current on/offratio, which is the ratio of the source-drain current in accumulationregime to the source-drain current in depletion regime.

At least five thin-film transistors were prepared with dimensions ofW=1,000 μm and L=5 μm. The following properties were obtained: Mobility:1.0-4.5 × 10⁻³ cm²/V · s Current On/off ratio 10²-10⁴.

EXAMPLE 2

(i) Synthesis of Small Molecular Thiophene Compound (II)

A solution of 0.5 g of precursor (10) as prepared in Example 1 in 15milliliters of chloroform was added to a mixture of 0.5 g of FeCl₃ and 5milliliters of chloroform in a 100-milliliter reaction flask. Thereaction mixture was stirred at 40° C. for 6 hours, during which time,the viscosity of the reaction mixture was observed to increase due toprecipitation of reaction product. Subsequently, the reaction mixturewas stirred at room temperature for 42 hours, and then diluted with 100milliliters of dichloromethane and washed with water. The organic phasewas separated and stirred with 150 milliliters of aqueous 7.5% ammoniasolution for 30 min. The mixture was then washed with water and pouredinto in 500 milliliters of stirring methanol. The precipitated productwas purified by Soxhlet extraction with methylene chloride and thenprecipitated from methanol, separated by filtration and dried in vacuoat room temperature overnight. Compound (II) was isolated from the crudeproduct by column chromatography on silica gel using an eluent solventsystem consisting of 95:5 of hexane:methylene chloride by volume, andfurther purified by recrystallized from isopropanol as a purple redcrystal in 40% yield, m.p., 67.2° C.

¹H-NMR(CDCl₃, ppm) δ 7.21-6.96 (m, 20), δ 2.83-2.77 (m, 12H), δ1.80-1.60(m, 12H), 1.50-1.20 (bs, 108H), 0.92-0.85 (t, 18H).

¹³C NMR (CDCl₃, ppm): δ 141.00, 140.31, 137.23, 137.09, 135.84, 135.48135.36, 135.30, 135.26, 130.70, 130.52, 129.91, 129.88, 127.03, 126.94,126.73, 124.37, 124.30, 124.28, 32.34, 31.07, 30.86, 30.09, 30.02,29.96, 29.94, 29.88, 29.78, 29.69, 23.11, 14.54.

(ii) Device Fabrication and Evaluation

A top-contact thin-film transistor structure, as schematicallyillustrated by FIG. 3, was used as the test device configuration. Thedevice was comprised of a n-doped silicon wafer with a thermally grownsilicon oxide layer of a thickness of about 110 nanometers thereon. Thewafer functioned as the gate electrode while the silicon oxide layeracted as the gate dielectric and had a capacitance of about 32 nF/cm²(nanofarads/square centimeter). The silicon wafer was first cleaned withargon plasma, methanol, air dried, and then immersed in a 0.1 M solutionof 1,1,1,3,3,3-hexamethyldisilazane in toluene for about 10 minutes atroom temperature. Subsequently, the wafer was washed with toluene,methanol and air-dried. The test semiconductor layer of about 30nanometers to about 100 nanometers in thickness was then deposited ontop of the silicon oxide dielectric layer by spin coating a 2 wt %solution of the compound (II) in chloroform at a speed of 1,000 rpm forabout 35 seconds, and dried in vacuo at 60° C. for 20 hours.

The devices were evaluated according to the procedure of Example 1. Atleast five thin-film transistors were prepared with dimensions ofW=5,000 μm and L=90 μm. The following properties were obtained:Mobility: 0.9-3.2 × 10⁻³ cm²/V · s Current On/off ratio: 10³-10⁴.

EXAMPLE 3

(i) Synthesis of Small Molecular Thiophene Compound (V)

Compound (V) was prepared according to the synthetic route depicted inScheme 2:

To a mixture of 1,4-benzenebis(pinacolboronate), (11) (0.1092 g, 0.331mmol) and monobromo-5,5′-bis(3-dodecyl-2-thienyl)-2,2′-dithiophene, (12)(0.5160, 0.692 mmol) in toluene (20 ml) under an argon atmosphere, wereadded tetrakis(triphenylphosphine)-palladium (0.020 g, 0.017 mmol),Aliquat336 (0.2 g in 5 milliliters of toluene) and 2M aqueous sodiumcarbonate (2 milliliters, 4 mmol). The mixture was then stirred at 100°C. for 24 hours. After the reaction, the mixture was diluted withtoluene (100 milliliters), washed with water, and dried with MgSO₄. Thesolvent was removed and the crude product (V) was isolated by columnchromatography on silica gel using an eluent solvent system consistingof 80:20 of hexane:methylene chloride by volume, and further purified bycrystallization from hexane and dichloromethane (50:50 by vol.),yielding an orange crystal in 71% yield, m.p., 90.8° C.

¹H-NMR(CDCl₃, ppm): δ 7.25-6.90 (m, 14H), δ 2.83-2.76 (m, 8H), δ1.75-1.60 (m, 8H); 1.41-1.20 (bs, 72H), 0.89-0.81 (t, 12H).

¹³C NMR(CDCl₃, ppm): δ 141.79, 141.27, 140.31, 137.19, 137.14, 135.81135.62, 135.50, 133.50, 130.70, 130.50, 126.94, 126.74, 126.61, 126.62,124.30, 124.27, 32.34, 31.07, 30.97, 30.08, 30.02, 29.94, 29.88, 29.88,29.78, 29.68, 23.11, 14.53.

(ii) Device Fabrication and Evaluation

A bottom-contact thin-film transistor test structure as described inExample 1 was used with the exception that Compound (V) was used insteadof Compound (I). The devices were first dried in vacuum oven at 60° C.for 20 hours, and then evaluated. At least five thin-film transistorswere prepared with dimensions of W=1,000 μm and L=5 μm. The followingproperties were obtained: Mobility: 0.7-1.6 × 10⁻³ cm²/V · s Currenton/off ratio: 10²-10³.

1. An electronic device comprising a semiconductor layer in contact witha number of electrodes, wherein the semiconductor layer includes a smallmolecular thiophene compound consisting of a plurality of thiopheneunits, each thiophene unit being represented by structure (A)

wherein each thiophene unit is bonded at either or both of the secondring position and the fifth ring position, wherein m is 0, 1, or 2,wherein each thiophene unit is the same or different from each other interms of substituent number, substituent identity, and substituentposition, wherein each R₁ is independently selected from the groupconsisting of: (a) a hydrocarbon group, (b) a heteroatom containinggroup, and (c) a halogen, wherein there is at least one thiophene unitwhere R₁ is present at the third ring position or the fourth ringposition, or at both the third ring position and the fourth ringposition, wherein for any two adjacent thiophene units as represented bystructure (A1):

there is excluded the simultaneous presence of the same or different R₁at the 3-position of one thiophene unit and at the 3′-position of theother thiophene unit.
 2. The device of claim 1, wherein thesemiconductor layer further includes a different small molecularthiophene compound consisting of a plurality of thiophene units and anoptional divalent linkage, with each thiophene unit of the differentsmall molecular thiophene compound represented by the structure (A). 3.The device of claim 1, wherein the hydrocarbon group is substituted. 4.The device of claim 1, wherein the hydrocarbon group forms part of aring structure attached to the third ring position and the fourth ringposition of the thiophene unit.
 5. The device of claim 1, wherein theheteroatom containing group forms part of a ring structure attached tothe carbon atoms at the third ring position and the fourth ring positionof the thiophene unit.
 6. The device of claim 1, wherein the number ofthiophene units is at least
 6. 7. The device of claim 1, wherein thenumber of thiophene units ranges from 6 to about
 32. 8. The device ofclaim 1, wherein the compound is symmetrical.
 9. The device of claim 1,wherein the compound is selected from the group consisting of:

or, a mixture thereof, where y is the number of units, wherein R and R′are the same or different from each other, R is selected from the groupconsisting of: (a) the hydrocarbon group, (b) the heteroatom containinggroup, and (c) the halogen, R′ is selected from the group consisting of:(a) the hydrocarbon group, (b) the heteroatom containing group, (c) thehalogen, and (d) a hydrogen.
 10. The device of claim 1, wherein thethiophene units are selected from the group consisting of:

wherein R₂ and R₃ are the same or different from each other, and areselected from the group consisting of: (a) the hydrocarbon group, (b)the heteroatom containing group, (c) the halogen, and (d) a hydrogen.11. The device of claim 1, wherein the device is an organic thin filmtransistor.
 12. The device of claim 1, wherein the compound is selectedfrom the group consisting of:

or a mixture thereof.
 13. A composition comprising: a small molecularthiophene compound consisting of a plurality of thiophene units, eachthiophene unit being represented by structure (A)

wherein each thiophene unit is bonded at either or both of the secondring position and the fifth ring position, wherein m is 0, 1, or 2,wherein each thiophene unit is the same or different from each other interms of substituent number, substituent identity, and substituentposition, wherein each R₁ is independently selected from the groupconsisting of: (a) a hydrocarbon group, (b) a heteroatom containinggroup, and (c) a halogen, wherein there is at least one thiophene unitwhere R₁ is present at the third ring position or the fourth ringposition, or at both the third ring position and the fourth ringposition, wherein for any two adjacent thiophene units as represented bystructure (A1):

there is excluded the simultaneous presence of the same or different R₁at the 3-position of one thiophene unit and at the 3′-position of theother thiophene unit.
 14. The composition of claim 13, further includinga different small molecular thiophene compound consisting of a pluralityof thiophene units and an optional divalent linkage, with each thiopheneunit of the different small molecular thiophene compound represented bythe structure (A).
 15. The composition of claim 13, wherein thehydrocarbon group is substituted.
 16. The composition of claim 13,wherein the hydrocarbon group forms part of a ring structure attached tothe third ring position and the fourth ring position of the thiopheneunit.
 17. The composition of claim 13, wherein the heteroatom containinggroup forms part of a ring structure attached to the carbon atoms at thethird ring position and the fourth ring position of the thiophene unit.18. The composition of claim 13, wherein the number of thiophene unitsis at least
 6. 19. The composition of claim 13, wherein the number ofthiophene units ranges from 6 to about
 32. 20. The composition of claim13, wherein the compound is symmetrical.
 21. The composition of claim13, wherein the compound is selected from the group consisting of:

or or, a mixture thereof, where y is the number of units, wherein R andR′ are the same or different from each other, R is selected from thegroup consisting of: (a) the hydrocarbon group, (b) the heteroatomcontaining group, and (c) the halogen, R′ is selected from the groupconsisting of: (a) the hydrocarbon group, (b) the heteroatom containinggroup, (c) the halogen, and (d) a hydrogen.
 22. The composition of claim13, wherein the thiophene units are selected from the group consistingof:

wherein R₂ and R₃ are the same or different from each other, and areselected from the group consisting of: (a) the hydrocarbon group, (b)the heteroatom containing group, (c) the halogen, and (d) a hydrogen.23. The composition of claim 13, wherein the compound is selected fromthe group consisting of:

or a mixture thereof.